This invention relates to the making of glass. More particularly, it relates to an improved method of melting and refining glass in a vertically elongated electric melting furnace.
Vertically elongated glass melting furnaces have peripheral heating electrodes have been known in the art of glass making for many years. Illustrative of the state of the art are the following references: U.S. Pat. Nos. 2,186,718 to Ferguson, 2,263,549 to Peyches, 3,524,206 to Boettner et al., 3,583,861 to Preston, 3,725,588 to t'Serstevens, 3,742,111 to Pieper and 3,755,606 to Boettner et al. and in several foreign patents Swedish Pat. No. 80,130, Italian Pat. No. 298,239, German (W) Pat. No. 736,937 and French Pat. No. 1,305,805.
The patents of Peyches, Boettner et al. and Preston show side-mounted electrodes at a plurality of elevations to heat molten glass in the illustrated furnaces and to heat and melt glass batch materials floating on the molten glass by radiation and convection through the molten glass and conduction from it to the unmelted batch materials. In Boettner et al. U.S. Pat. No. 3,524,206 the heat energy applied to a furnace at any given elevation is greater than the heat energy applied at any lower elevation. In Preston the heat energy applied at a lower elevation of a furnace is sufficient to create an upward or countercirculation of glass particularly at the periphery of the furnace so that the downward flow of glass from an upper portion at a higher elevation of the furnace is nullified. The patent of Boettner et al. includes a scheme for increasing contact between unmelted batch and molten glass by encouraging the bottom surface of the floating glass batch to assume a conical shape similar to the depressed shape shown in U.S. Pat. No. 2,397,852 to Gentil. The German patent shows outwardly-tapered side walls near the top of a furnace, and the patent of Preston illustrates inwardly tapered side walls near the bottom of a furnace.
Crossfiring electrodes have been considered in the past for use in melting glass. The following references disclose plural electrode arrangements and the use of multiphase electric power to provide intersecting electric current paths through molten glass. High current densities at the current path intersections remote from the electrodes themselves result in hot spots at those locations of high current densities as described in these references.
U.S. Pat. Nos. 2,018,883, 2,018,884 and 2,018,885 of Ferguson all describe crossed electrode pairs in shallow dish-like furnaces for directing electric currents along intersecting paths across the furnaces. U.S. Pat. No. 2,018,883 is directed to an electric furnace having means for passing a plurality of independent electric currents through its contents along diametrical paths wherein the furnace is deeper at its center than about its periphery. U.S. Pat. No. 2,018,884 is directed to a similar furnace having means to pressurize the interior of the furnace and thereby control the flow of molten glass therefrom. U.S. Pat. No. 2,018,885 is directed to a similar furnace further having means for applying magnetic forces across the electric currents directed through it.
U.S. Pat. Nos. 2,984,829 and 2,993,079 of Augsburger describe glassmaking furnaces provided with heating electrodes to direct electric currents along intersecting paths through molten glass contained in them. The described furnaces appear to be relatively shallow compared with their horizontal dimensions.
U.S. Pat. No. 3,852,509 of Rutledge et al. describes a vertically elongated furnace having a plurality of electrodes about its periphery. Opposing electrodes, both diametrically opposing and peripherally opposing, are energized together to direct electric currents through molten glass in the furnace. Electric current path intersections occur, not only at the center of the furnace, but about the whole furnace including near its periphery and between its center and periphery.
All of these patents describing crossfiring electrodes describe the use of electric transformers to yield the requisite number of currents of differing phases to accommodate the particular number of electrodes employed in each scheme.
While the glassmaking furnaces described in the prior art are all apparently useful for the melting of glass batch to prepare molten glass, the vertically elongated glassmaking furnaces of the past with their heating electrodes are not believed to be as thermally efficient as desired, nor to refine molten glass sufficiently to provide for the making of high-quality flat glass. It is an objective now to provide an improved method of operation for vertically elongated, electrically heated glassmaking furnaces and to provide certain improvements to such furnaces themselves.